sparseirq: fix !SMP && !PCI_MSI && !HT_IRQ build

Ingo Molnar wrote:

>>>  drivers/pci/intr_remapping.c: In function 'irq_2_iommu_alloc':
>>>  drivers/pci/intr_remapping.c:72: error: 'boot_cpu_id' undeclared (first use in this function)
>>>  drivers/pci/intr_remapping.c:72: error: (Each undeclared identifier is reported only once
>>>  drivers/pci/intr_remapping.c:72: error: for each function it appears in.)

sparseirq should only be used with SMP for now.